1. Field of the Invention
The present invention relates to aralkyl substituted phenolic novolak resins, particularly aralkyl substituted resorcinol novolak resins which are used in rubber compounding resins to give improved mechanical properties and improved tire cord to rubber adhesion.
2. Brief Description of the Prior Art
In the manufacture of reinforced rubber products, such as automobile tires, it is important to have good adhesion between the rubber and the reinforcing material. Originally, adhesion of the rubber to the reinforcing material was promoted by pretreating the reinforcing material with certain adhesives. This proved unsatisfactory and it is now conventional to incorporate into the rubber during compounding various chemicals that react to improve the adhesion of the reinforcing materials and rubber during the vulcanization process. This compounding adhesion method is now generally practiced irrespective of whether the reinforcing materials are pretreated with adhesives or not.
The conventional method of compounding adhesion comprises compounding into the rubber before vulcanization a two part adhesive system. One part is a methylene donor compound that generates formaldehyde upon heating. The other part of the adhesive system is a methylene acceptor compound. During the vulcanization step the methylene donor upon heating releases formaldehyde and methylene acceptor reacts with the formaldehyde, rubber and reinforcing material with a resultant increase in adhesion of the rubber to the reinforcing materials. In addition, proper selection of the methylene donor and methylene acceptor can improve many other properties of the final product. The methylene donor and the methylene acceptor are compounded into the rubber and thus have a significant effect on the process of making the reinforced rubber product.
The methylene donor compounds are compounds that generate formaldehyde upon heating. Examples of the most commonly used compounds are hexamethylenetetramine and the various methyol melamines. There are many other suitable methylene donor compounds that may be used. Examples are set forth in U.S. Pat. No. 3,751,331.
Many different methylene acceptor compounds have been tried with various degrees of commercial success. Examples of the most common methylene acceptor compounds are resorcinol, resorcinol formaldehyde novolak resins, phenol formaldehyde novolak resins and phenol resorcinol formaldehyde novolak resins. While all of these methylene acceptor compounds will increase the adhesion of the rubber to the reinforcing materials, they have significant drawbacks.
As mentioned, resorcinol, resorcinol formaldehyde novolak resins and resorcinol phenol formaldehyde novolak resins have been used in the rubber compounding industry as methylene acceptors. These resorcinol containing compounds are unique materials for rubber compounding since they are not only good adhesion promoters but also act as thermosetting or vulcanizing plasticizers.
The thermosetting properties of resorcinol and resorcinol containing resins on curing yield a vulcanizate that has increased hardness, better abrasion resistance and improved aging resistance. The vulcanizate also has better solvent and oil resistance, is stiffer and gives improved finishes to the cured rubber. This combination of adhesion promotion and plasticizing makes the resorcinol containing methylene acceptors preferred compounds. In addition, as plasticizers the resorcinol containing compounds allow easier processing, higher loading and excellent extrusions for the rubber compounds. Even though resorcinol and resorcinol containing novolak resins provide improved adhesion, improved mechanical properties and improved processibility they are not without their drawbacks. For example the fuming and hygroscopicity of resorcinol containing methylene acceptors present problems in rubber compounding and handling.
To overcome these drawbacks manufacturers have been seeking modified resorcinolic derivatives that do not produce volatiles such as resorcinol at Banbury temperatures, are not hygroscopic and do not bloom. Several such compounds have been suggested.
U.S. Pat. No. 4,889,891 discloses an alkyl substituted resorcinol formaldehyde novolak. U.S. Pat. No. 4,605,696 discloses the use of resorcinol monoesters, particularly resorcinol monobenzoate. In U.S. Pat. No. 4,892,908 there is disclosed the use of keto derivatives of resorcinol such as benzoylresorcinol. In a co-pending application U.S. patent application Ser. No. 452,979, it is suggested that certain alkyl substituted phenol resorcinol formaldehyde novolak resins can be used to overcome the drawbacks of resorcinol.
Even though these resorcinol containing methylene acceptors may be useful there is always a need for improved methylene acceptor compounds. This is especially true of methylene acceptor compounds that provide a wide variety of desirable properties.
It is therefore an object of this invention to provide an improved methylene acceptor for use in rubber compounding. More particularly, it is the object of this invention to provide an improved methylene acceptor based on aralkylated novolak resins.